1. Technical Field of the Invention
The present invention generally relates to telecommunications. More particularly, and not by way of any limitation, the present invention is directed to a clock synchronization system and method for a scalable telecommunications node disposed in an access network.
2. Description of Related Art
The remote access market is undergoing a major metamorphosis. Three factors serve as catalysts for change. The first is the growing number of users, for example, small office/home office (SOHO) users, demanding high performance Internet and remote access for multimedia. Liberalized governmental activity with respect to telecommunications is another factor, which is fostering broader competition through deregulation in local area markets everywhere. The third and final factor is congestion in the Public Switched Telephone Network (PSTN), originally designed and developed for voice-only traffic.
There have been several important advances in telecommunications technology that enable high rates of throughput in carrier networks' backbone connections. For example, by implementing Asynchronous Transfer Mode (ATM) networking technology over a Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH) physical layer, carrier networks can achieve data rates of up to several hundred megabits per second (Mbps). However, efforts to meet the bandwidth demand for remote access have been beset by the limitations of the existing twisted-pair copper cable infrastructure (i.e., access network) provided between a carrier's central office (CO) and a subscriber's remote site, typically referred to as the local loop. In the telecommunications art, these limitations are sometimes collectively described as the “last-mile” problem.
Current access network solutions that attempt to avoid the bottleneck created by the last-mile problem involve the use of fiber optic technology in the local loop also. As with the high-speed carrier networks, the fiber-based local loop infrastructure is typically architected using SONET as the physical layer technology. With recent developments in optical components and related opto-electronics, in addition to improvements in network design, broadband access is now becoming commonplace.
Moreover, coupled with the phenomenal growth in popularity of the Internet, there has been a tremendous interest in using packet-switched network (PSN) infrastructures (e.g., those based on Internet Protocol (IP) addressing) as a replacement for the existing circuit-switched network (CSN) infrastructures used in today's telecommunications networks. From the network operators' perspective, the inherent traffic aggregation in packet-switched infrastructures allows for a reduction in the cost of transmission and the infrastructure cost per end-user. Ultimately, such cost reductions enable the network operators to pass on the concomitant cost savings to the end-users.
Accordingly, a new breed of service-centric networks (distinct from the existing voice-centric and data-centric networks) are being explored for implementation on what is known as the next-generation network (NGN) infrastructure, where integrated voice/data/video applications may be provisioned using a packet transport over a packet network in an end-to-end transmission path. As alluded to hereinabove, it is believed that using a packet network infrastructure in access networks provides higher transmission efficiency, lower operation and maintenance costs, and a unified access.
Traditional access systems allow accessing a digital local voice switch, such as a Class 5 switch, by extending a plurality of metallic loops and aggregating them in a bundle for efficiently transmitting the time-division multiplexed (TDM) voice traffic. Typically, such access networks are architected using one or more access nodes in a variety of configurations, e.g., point-to-point chains, rings, etc. Each access node itself comprises several channel banks that provide line interfaces servicing a large number of subscribers. In order to be able to meet projected subscriber growth and associated demand for services, the access nodes are typically over-designed in the sense that a high number of channel banks (e.g., upwards of 8 banks or more) are provisioned for each node at the outset. Additionally, the internal architecture of a conventional access node is such that the banks are interconnected in a “star” configuration, with a centralized “head star” bank coupled to a plurality of satellite banks (i.e., a hub-and-spoke arrangement).
Those skilled in the art should recognize that the conventional “full-provisioning” approach to implementing an access network is beset with several disadvantages. First, the up-front cost of setting up the network can be prohibitively high. Moreover, the increased cost structure will be particularly inefficient where only a small number of subscriber lines need to be supported initially, especially in an access node with a star architecture that is not fully populated. In addition, unrelated to the cost structure factors, the implementation of most conventional access nodes is fraught with internal inefficiencies also, as the local data traffic within the node requires unwieldy protocol conversions. Also, the local links for effectuating bank-to-bank communications are not capable of supporting the bandwidth rates necessary to implement value-added, advanced services that can be provisioned in the NGN.
In addition, regardless of whatever architectural advances are implemented in order to address the concerns set forth above, certain timing and frame alignment issues must also be resolved. For instance, switching TDM traffic at DS0 level requires a fairly robust synchronization scheme, especially where massive throughput rates are required, so that data frames that need to be switched are properly aligned to ensure error-free switching. Accordingly, conventional timing and frame alignment schemes employed in an access node today require a large amount of storage because of the inherent latency in having to wait for longer periods before the frames can be aligned at appropriate times. It should be appreciated that such schemes are not only memory-intensive (an expensive proposition), but they also introduce additional delays in the timing chain of the node which negatively impacts the overall throughput of the node. These and other drawbacks are further exacerbated when the interbank cabling issues are also involved.